The present invention is generally related to a probe interface assembly, and, more particularly, to a compliant probe interface assembly for imaging breast tissue with an ultrasound probe in conjunction with X-ray mammography, and thereby provide geometrically registered X-ray and ultrasound images.
FIG. 1 illustrates a schematic of conventional X-ray mammographic equipment combined with an ultrasound scanning system. This usually includes a compression plate 10 (e.g., a plate made up of polycarbonate such as Lexan or other suitable materials) to compress and flatten breast tissue 12 against a detector plate 18. In order to perform an ultrasound scan subsequent to an X-ray mammogram, the system further includes an ultrasound probe 14, as may be positioned to traverse atop such a plate to generate ultrasonic images of the internal structure of the breast tissue. A gel 16 may be provided as shown in FIG. 1. The gel typically comprises a suitable composition for reducing acoustic impedance mismatch and reflectance at the plate/probe interface. Ideally, the compression action should provide uniform contact of the breast tissue with the compression plate to achieve appropriate ultrasound propagation as well as superior X-ray imaging.
In practice, the compressing surface of the compression plate may deform when exposed to typical mammographic breast compression forces. The resulting maximum deflection of the plate, as may be measured from a horizontal plane, should be typically constrained to lie within 1 cm, as per MQSA requirements. Since the ultrasound probe 14 rides on top of this deformed plate, as shown in FIG. 1, the ultrasonic beam propagates through a non-uniform gap and a non-parallel surface.
A varying gap changes the ultrasound path between the probe and the compression plate and leads to inconsistent attenuation. A non-parallel surface may lead to variable beam refraction, as the ultrasound beam may be formed from multiple elements in a linear array ultrasound probe. Each of these conditions could have adverse effects on the ultrasound image quality. These conditions may also make it burdensome for a radiologist to correlate an X-ray image to an ultrasound image due to the lack of a consistently reproducible setup from one scan to the next scan.
To avoid such adverse effects, it would be desirable to maintain a uniform gap and/or parallel alignment between the probe face and the compression plate. For example, this would allow keeping a desired profile of ultrasound beam incident angles on the compressive plate so that corrective time delays, as may be generated by a beam-forming processor during transmit and receive modes, will correctly adjust the beam summation to correct for refractive effects through the compression plate.
Early attempts to use ultrasound for breast imaging involved conventional handheld scanning of the free uncompressed breast. Since such a handheld ultrasound imaging does not provide geometric registration with the X-ray images, it is difficult to compare the features obtained in each image since these may belong to different regions of the breast. Hence, there is a need for a system that performs X-ray and an automated ultrasound scan in a single patient setting.
Known automated scanning systems have failed to effectively solve the issues arising from a varying gap and a non-parallel surface. One known attempt was to position the ultrasound probe at the farthermost location (height) from the deformed plate. Although this positioning would ensure that the deformed plate would not obstruct the probe during the course of the scan, the resulting variation in probe-plate distance along the face of the transducer would deteriorate image quality. One conceptual possibility would have been creating a compression plate constructed of special materials and/or geometry to be sufficiently rigid to avoid deformation in the presence of compression loads, and at the same time be sufficiently radio- and sono-lucent to allow passage to the x-rays and ultrasound beams. In practice, however, cost-effective materials and plate configurations suitable for a combined mammographic and ultrasonic examination will inevitably bow to some degree.
Another drawback of known automated scan systems is that they do not have the flexibility of a handheld system to orient the probe at any desired orientation with respect to a region of interest and at the same time maintaining contact with the bowed compression plate. An ultrasound transducer could have a higher resolution along its lateral axis as compared to its elevation axis. Hence, it would be desirable that the probe lateral axis be aligned nominally with the direction of orientation of the specific structure that needs to be resolved.